Aromatics, especially light aromatics such as BTX (benzene, toluene, and xylenes) are important basic organic materials. With the continuous development of global industry and economy in recent years, the demand of aromatics increases continuously. Currently, aromatics mainly come from the naphtha reforming process and the pyrolysis gasoline hydrogenation process, both of which use petroleum as the raw material. According to statistics, aromatics produced by using petroleum as the raw material account for more than 90% of the total aromatics around the world.
However, the cost of aromatics production from petroleum rises sharply due to continuous consumption and increase in price of petroleum resources. Therefore, in the long run, the production of aromatics through the naphtha reforming process and the pyrolysis gasoline hydrogenation process may not meet the growing requirements for aromatics and the cost is higher.
Comparing with petroleum resources, coal resources in the world are more abundant so that they can provide plenty of raw materials for the coal chemical industry. So far, the methanol production process using coal as raw material has been mature. At the same time, excess production capacity of methanol on a global scale has become a serious problem. Therefore, directly converting methanol to produce aromatics may be regarded as a very promising aromatics production line.
U.S. Pat. No. 4,686,312 relates to a catalyst and a process for converting C1-C4 oxygenated hydrocarbons to aromatics. The process is two-stage reactions comprising a primary stage, in which methanol-enriched feedstock is contacted with HZSM-5 zeolite and then converted into an intermediate product containing predominantly aliphatic hydrocarbons, and a secondary stage, in which the intermediate product from the primary stage is contacted with metal modified ZSM-5 zeolite to generate a product rich in aromatics.
US2002/0099249 relates to a process for converting methanol to aromatics and a hybrid catalyst system used. In the process, a feedstock comprising methanol is converted to aromatics by sequentially contacting the feedstock with a first catalyst containing silicoaluminophosphate (SAPO) and then a second catalyst containing metal modified ZSM-5 zeolite. Both of the catalysts contain certain amount of binders.
US2010/0234658 discloses an aromatization catalyst in the type of multi-metal loaded zeolite molecular sieve. The catalyst comprises La and a molecular sieve, a binder, and at least one element selected from Mo, Ce and Cs. In an example of this invention using methanol as the reactant, in the conditions of 450° C., ambient pressure and an WHSV of 9 h−1, the yield of aromatics and BTX (methanol weight based) are up to 18.8% and 13.7% respectively, i.e., 43.0% and 31.5% respectively based on carbon.
CN1880288A discloses a process for converting methanol to aromatics and a catalyst used. The catalyst is prepared by mixing a supporter of small particle size ZSM-5 zeolite with a binder (pseudo-boehmite, gamma-alumina or diatomaceous earth) and then molding the mixture, followed by loading active components of Ga and La. The content of the binder in the catalyst may be in a range of 14˜34% by weight. In the conditions of 300˜460° C., 0.1˜5.0 MPa and 0.1˜6.0 h−1 (WHSV of the liquid feedstock), methanol is contacted with the above catalyst. The yield of aromatics in the formed product is higher than 31% (based on the weight of methanol), i.e. higher than 72% based on carbon. However, this process needs two stages to increase the total yield of aromatics, in which low carbon hydrocarbons obtained from a first stage are subjected to a second reactor to proceed with the aromatization. Thus, this is a rather complex process with many separation steps.
CN101244969A relates to an equipment and a method for a continuous aromatization and catalyst reactivation, specifically relates to a fluidized bed equipment for aromatization of C1˜C2 hydrocarbons or methanol and catalyst reactivation and the operational approach thereof. The catalyst used in this invention consists of three components, i.e. a molecular sieve, a metal and a structure stability agent or strengthening agent (corresponding to a binder), wherein the content of the structure stability agent or strengthening agent is more than 20%. It recites in this invention that when an aromatization using methanol as raw material is operated, 97.5% of conversion of methanol, 72% of single pass yield of aromatics (carbon-based) and 55% of BTX selectivity are obtained. Although a high yield of aromatics is obtained, the methanol conversion is low and furthermore the selectivity of BTX that is most valuable in the aromatics is only 55% or so.
As mentioned above, molded catalysts for the conversion of methanol to aromatics reported at present usually consist of a zeolite molecular sieve such as ZSM-5, ZSM-11 or MCM-22, a binder and an active component and a modified component for dehydrogenation. The binder is generally an amorphous oxide such as aluminum oxide, silicon oxide, etc. Since there are certain amounts of binders in these molded catalysts, a portion of pores of the zeolite molecular sieve are blocked up, which inhibits the diffusion of reactants and products and finally results in dropping of activity of the catalyst and selectivity of the subject product. The active component and the modified component for dehydrogenation are usually loaded onto the catalyst by impregnation or mechanical mixing method. However, the active component and the modified component are distributed in homogeneously, most staying on the surface of the catalyst while only a little entering into the inside of the catalyst when they are loaded by using these conventional methods.
In the current situation that the methanol production process using coal as raw material has been mature and the production capacity of methanol may be excess, how to promote the properties of catalysts for converting methanol to aromatics, for example how to promote the activity of the catalysts or simplify the preparation process, may be a main development direction in the field.